Liquid toner compositions for use in liquid toner imaging systems normally comprise a carrier liquid and toner particles. These two components deplete at different rates from a liquid toner supply reservoir which is normally part of such systems. The relative component depletion rates are dependent on the percent coverage of the images produced by the imaging system and on other factors.
Imaging systems, be they printers or copiers, generally produce a variety of images having a wide range of print coverage. In general, the relative depletion rates of the various components of liquid toner will depend on the print coverage. It is well known that the balance between the various components of a liquid toner can have a strong effect on the quality of printed images, therefore most imaging systems have replenishment systems. These systems include replenishment with toner concentrate, having a relatively high percentage of particles and also containing carrier liquid, and with carrier liquid free of toner particles. One or both of these replenishment components may have charge director added thereto, or charge director may be supplied in a separate charge director replenishment solution.
Toner concentrate is added whenever the liquid toner becomes depleted of toner particles. The concentration of toner particles may be determined by measuring the optical density of the liquid toner composition in the reservoir. Carrier liquid is supplied whenever the total amount of liquid toner in the reservoir falls below a certain level. Charge director may be added when the conductivity of the solution is reduced.
An exemplary system for the replenishment of liquid toner components is described in U.S. Pat. No. 4,860,924 the disclosure of which is incorporated herein by reference.
The liquid carrier supply generally includes apparatus for the measurement of the liquid level in the reservoir, and a series of pumps and or valves which are operated in response to a signal from the measurement system to replenish the carrier liquid in the reservoir by pumping or otherwise transporting carrier liquid from the carrier liquid replenishment supply.
In color systems, liquid toners of different colors are required, each having a separate replenishment system for toner particle concentrate and for carrier liquid, including separate measurement and supply systems. These separate systems add to the expense and complication of such systems and reduce their reliability.